Normally in a switch mode power supply (SMPS) the peak current value is related to the average current in the inductor. But with some SMPS circuit topologies, when the pulse width modulation (PWM) duty cycle is greater than 50 percent there is a changing relationship between the peak and the average current through the inductor, where the peak current is no longer proportional to the average current. This may cause instability, e.g., sub-harmonic oscillation. The basic cause of this instability (sub-harmonic oscillation) is that the inductor current does not return to zero by the start of the next PWM cycle. To prevent this instability many SMPS applications use a control methodology called “Peak Current Mode Control” (PCMC). PCMC sets a maximum or peak level for the inductor current. The SMPS power inductor current may be monitored with an analog comparator. The threshold of this comparator sets the peak current limit of the SMPS inductor. Referring to FIG. 1, the peak current is related to the average current (with some assumptions). For digital implementations of PCMC applications, a digital-to-analog converter (DAC) is used to provide the threshold voltage to the comparator.
PCMC modulates a downward slope onto the current reference value to stabilize the power circuit. This scales the peak inductor current with a decreasing reference slope as the PWM cycle progresses. The basic idea to limit the circuit to a peak current level that can be properly discharged before the next PWM cycle begins.
Referring to FIG. 2, depicted is a schematic block diagram of a prior technology analog slope compensation circuit used in a SMPS. The reference voltage used for peak current detection is reduced as the user's PWM duty cycle duration increases. Referring to FIG. 3, depicted is a schematic block diagram of a portion of the prior technology analog slope compensation circuit shown in FIG. 2. There are a number of issues when using analog slope compensation. For example, analog circuits are expensive. Analog circuit implementations require external components for configuration, or need highly adaptable internal function modules in the PWM controller integrated circuit. The analog slope generator has a limited voltage range. In addition, in small process geometry integrated circuits the necessary analog circuits required for slope compensation functionality are too large and expensive to build and are therefore not cost effective for use in price sensitive products.